The Little Green Plant That Could: Duckweed as a Renewable and Sustainable Biofuel Feedstock

To meet the energy demands of a booming global population and preserve biosphere integrity, we must reduce our dependency on fossil fuels and find a more cost-effective and sustainable energy source. The development of renewable biofuels will aid in overcoming these daunting economic and environmental challenges. Corn, the main biofuel feedstock in the United States today, has serious drawbacks as an energy source; its high-maintenance cultivation requires significant amounts of fertilizer and freshwater while competing with food crops for arable land. In contrast, the small aquatic plant duckweed has emerged as a highly promising biofuel feedstock. Its composition—high starch and low lignin—is ideal for ethanol production. Furthermore, duckweed does not compete with land crops, grows rapidly, and is easily harvestable. We are actively constructing a demonstration pipeline to assess the potential of duckweed as a renewable and sustainable crop for ethanol production. Liquid sewage is used as an inexpensive fertilizer source to support duckweed growth, and post-harvest, the cleaned wastewater can be recycled. Optimization of this new approach involves screening duckweed plants collected from around the world to find a strain that is best suited for ethanol production. However, morphological identification of duckweed species and ecotypes can be difficult, as many appear similar or identical. Thus, we are employing DNA-based methods in the laboratory to develop reliable methods of distinguishing duckweed strains. These efforts will lay the groundwork for future experiments, allowing us to gain much needed insight into basic duckweed biology.

The Little Green Plant That Could: Duckweed as a Renewable and Sustainable Biofuel Feedstock

To meet the energy demands of a booming global population and preserve biosphere integrity, we must reduce our dependency on fossil fuels and find a more cost-effective and sustainable energy source. The development of renewable biofuels will aid in overcoming these daunting economic and environmental challenges. Corn, the main biofuel feedstock in the United States today, has serious drawbacks as an energy source; its high-maintenance cultivation requires significant amounts of fertilizer and freshwater while competing with food crops for arable land. In contrast, the small aquatic plant duckweed has emerged as a highly promising biofuel feedstock. Its composition—high starch and low lignin—is ideal for ethanol production. Furthermore, duckweed does not compete with land crops, grows rapidly, and is easily harvestable. We are actively constructing a demonstration pipeline to assess the potential of duckweed as a renewable and sustainable crop for ethanol production. Liquid sewage is used as an inexpensive fertilizer source to support duckweed growth, and post-harvest, the cleaned wastewater can be recycled. Optimization of this new approach involves screening duckweed plants collected from around the world to find a strain that is best suited for ethanol production. However, morphological identification of duckweed species and ecotypes can be difficult, as many appear similar or identical. Thus, we are employing DNA-based methods in the laboratory to develop reliable methods of distinguishing duckweed strains. These efforts will lay the groundwork for future experiments, allowing us to gain much needed insight into basic duckweed biology.

21733 Views

Share Presentation

Will the approach using duckweed have scale economies i.e., will it permit duckweed to be processed on site, or will it require it to be collected from many sources and processed centrally or will it be better off being grown in one central location ? What are the associated scientific challenges ?

Thanks for your question, Dr. Iyer. The scaling-up of duckweed-based biofuels will depend largely on developing a cheap and efficient method of drying the plant material on-site. Duckweed has a water content of 92-95%, making transportation of wet biomass economically unfeasible. Sun drying is generally inefficient while heat drying requires a large input of energy. Mechanical squeezing of the plant tissue may effectively remove most water at low cost but has yet to be implemented on a large scale. Ideally, duckweed would be grown on open ponds, as opposed to within greenhouse enclosures, to minimize costs. Such an approach, however, demands careful consideration of many environmental factors, including climate and pests. To maximize throughput and yield, duckweed could be grown and harvested from multiple sources, dried on-site, and then transported to a central location for downstream processing.

Thanks for your inquiry, Dr. Clancy. Strains with high starch provide the most readily available source of fermentable sugars, maximizing biofuel yield. Plants contain two main reservoirs of fermentable sugars: cellulose and starch. Cellulose is packed into tough fibers that require harsh chemical conditions at high temperatures to break down into glucose sugar units. Starch, on the other hand, is easily extracted from plant material and directly hydrolyzed into glucose by simple enzymatic reactions. The resulting glucose is fermented by yeast into ethanol and then distilled. Subsequently, the ethanol can be blended with gasoline at variable percentages (E10, E15, E85) for use in vehicles. This overall conversion of starch into ethanol can be a highly efficient process; up to 95% yield has been observed.

However, a duckweed strain with high starch content is not the only important criterion. We need to find a strain that balances starch accumulation with a fast growth rate to maintain a steady, high yield under a particular set of growing conditions (e.g. pH, light intensity, temperature). These two metrics—high starch content and rapid growth rate—have the highest priority in our screening of duckweed strains for biofuel production.

Yes, there is. A published report (Xu et al. 2011) previously identified a strain of Spirodela polyrhiza with a high growth rate (12.4 g dry weight/m^2/d) and high starch content (31% dry weight, d.w.) grown on dilute swine wastewater. Using this strain, the researchers were able to obtain an ethanol yield of 6.42 × 10^3 liters/hectare, which is approximately 50% higher than that typically achieved using corn.

A preliminary screen conducted by our lab identified three S. polyrhiza strains and one S. intermedia strain that were promising. The S. polyrhiza strains had growth rates of 17.86-20.94 g d.w./m^2/d and starch contents of 18.5-27.6% d.w. The S. intermedia strain had a growth rate of 6.57 g d.w./m^2/d with a 42.6% starch content. Plants were grown under sterile laboratory conditions. We aim to select a strain that has both a good starch content and fast growth rate.

Could you comment on the scalability of this approach? What is the conversion efficiency of energy from sunlight to ethanol for duckweed? If the averaged solar insolation (flux) is 200-300 W/m^2 how much water surface area would you need growing duckweed to produce annually, say, a million liters of ethanol? Given the current ~53 billion liter/year production of ethanol in the US, will there be sufficient wastewater and underlying land available for duckweed biomass conversion to support a reasonable fraction of our ethanol production needs?

Thanks for your question, Dr. Harrison. The U.S. produces roughly 40 billion gallons of municipal wastewater per day (US Wastewater Treatment Factsheet, University of Michigan, http://css.snre.umich.edu/css_doc/CSS04-14.pdf). Assuming all of that wastewater is used for growing duckweed, 25,000 hectares of 0.6 m-deep ponds would be filled daily (9 million ha yearly). Recently, Xu and colleagues (2011) obtained an ethanol yield of 6420 liters/hectare/year by growing duckweed on wastewater. Using this yield for the sake of simplified calculations, I estimate that roughly 1.6 million liters of ethanol per day, or 60 billion liters per year, could be generated from US municipal wastewater, exceeding current US ethanol production. In addition, the 9 million hectares required for these duckweed-wastewater systems is less than the 14 million hectares of land devoted to cultivating corn for ethanol production (FAOStat, 2010; Sci Am, Feb 2013). Thus, based on these rough estimates, duckweed would indeed be a viable ethanol production platform for our current needs.

In your test cases, you are using waste water that has a relatively high fertilizer content. This brings up two issues if one is to assume duckweed is to supplant corn and soybeans as a major biofuel crop: (1) How much production can be achieved with existing wastewater ponds that have enough fertilizer? (2) Assuming that production needs exceed wastewater ponds, how much fertilizer is needed in comparison to soybeans and corn? As you are probably aware, a tremendous amount of energy goes into fertilizer production. Therefore, fertilizer use can have a dramatic impact on the energy balance of the overall process.

Hi, Dr. Yates. Yes, reducing or eliminating the need for fertilizer would be a game changer for the renewability and sustainability of biofuel feedstocks. That is why it is so imperative to integrate either agricultural or municipal wastewater as a replacement for conventional, industrially made fertilizers. Recycling the wastewater for crop irrigation or aquaculture systems are added benefits. To address your first question, we have obtained an ethanol yield of 2,407 liters/hectare/year at our local test site—assuming a 6-month growing period, 30% starch duckweed by dry weight, and a fermentation efficiency of 95%. As this was our first attempt, there is much room for improvement. Another group (Xu et al. 2011) calculated an ethanol yield of 6,420 liters/hectare/year.

For the second question, we do not know the amount of fertilizer necessary if production needs exceed wastewater ponds. A visiting chemical engineering student from South Africa (sponsored through our IGERT’s international collaboration!) is currently quantifying the duckweed biomass accumulation per unit of fertilizer. Another option is to build open ponds to accommodate the need and fill them with wastewater from existing facilities, since there is an abundance of municipal wastewater, as I mentioned in my response to Dr. Harrison’s question, as well as the untapped potential of agricultural wastewater. We have also been approached by farmers who want to use fallow ponds on their farms for duckweed production.

Excellent video! I am going to tell others about this project, One question:
How much biomass/m^2? It seems to me (watching wild duckweed) that this might be a challenge – whcy maybe needs to wait until you have the whole pipeline prototyped. Duckweed shade each other out, I think. One advantage of some algae-based systems I’ve seen is that side-illumination of columns enables more biomass per cubic whatever – on the other hand, duckweed may be great for “waste spaces” like the demo pond you show?
Good luck with this!

Thanks for your question, Dr. Drayton! The best performing species from a preliminary screen in our lab have growth rates from ~18-21 g dry weight/m^2/d under sterile lab conditions. Another paper reports a different duckweed species to reach 30 g d.w./m^2/d in lab conditions. As you mentioned, duckweed in the wild is a different story. They often grow as multiple genera/species together. The smaller duckweeds fill in the spaces on the water’s surface that the larger duckweeds are not able to occupy. On open ponds, the duckweeds can also form thick (multi-layer) mats.

In contrast, algal systems are expensive and energy-intensive. The algae need to be churned frequently for maximal photosynthesis in closed bioreactors. In addition, algae are difficult to harvest due to their microscopic size.

Wenqin Wang

Guest

May 22, 2013 | 12:17 a.m.

It is well done. Duckweeds, like you, are so cute!

Michael Lawton

Guest

May 22, 2013 | 12:54 a.m.

Excellent presentation on this novel approach to generating biofuels and remediation water. Good luck with your research and thanks for sharing this with the community. Maybe this is the start of a Duckweed Dynasty.

Massimo Bosacchi

John Cross

Guest

May 22, 2013 | 11:50 a.m.

Dear Ms Chu,

I saw your nice video posted on the IGERT website, and would like to place a link to it on my educational website, The Charms of Duckweed, hosted by the Missouri Botanical Garden (Mobot). If the IGERT website is a temporary location, do you have a permanent site for it, or may I copy it to the Mobot server?

Also, one sequence that is particularly nice is the time-lapse of duckweed growing. If possible, could you make that available as a stand-alone video sequence? I would like to feature it on the Charms of Duckweed.

We could potentially use municipal and agricultural wastewater to support biomass growth. It would be a great way to treat the wastewater for downstream applications like crop irrigation and aquaculture. It’s a cheap fertilizer source!

This is fantastic. Good job on this one. Does duckweed offer any advantages over Chlamydomonas reinhardtii? I am by no means an expert in the field, but this is a hot topic in plant biology so I’m always excited to see this kind of stuff. You really had great production on the video too. I hope to see more of this in the future, good luck making this energy positive!

Hi Geoffrey. Thanks for your question. One of the advantages of open pond duckweed cultivation is disease resistance. Chlamy cultures can be easily contaminated and are prone to attacks by bacteria, viruses, and other microalgae when grown on open ponds. The duckweed ponds that I have visited do not appear to have significant issues with disease or pests. There is very little literature that addresses diseases of duckweed. Another major advantage of a duckweed-based biofuels system is the ease of harvesting. Since duckweed floats, it is easier to harvest than algae which reside throughout the water column in open ponds. We can harvest the duckweed by simply skimming the pond surface. In contrast, harvesting algae typically involves flocculation. Flocculating agents are usually very expensive, especially for industrial-scale production, and so drive up the cost of algal biofuels. In addition, the algal sludge has limited use in downstream applications. Another difference between duckweed and Chlamy is the desired end product. As far as I know, algal biofuel research is primarily focused on lipid (oil) production for biodiesel, whereas our goal is starch production (however, oil production in duckweeds may also be feasible).

Tamra Fakhoorian

Guest

May 27, 2013 | 12:11 p.m.

Philomena, great job on the video and congratulations on the awards! I know I speak for all our International Lemna Association members when I say, “You are going to go great places in the duckweed industry!”